Question: Simplify the following expression: $r = \dfrac{2}{9z} + \dfrac{1}{4z}$
Explanation: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9z$ and $4z$ $\lcm(9z, 4z) = 36z$ $ r = \dfrac{4}{4} \cdot \dfrac{2}{9z} + \dfrac{9}{9} \cdot \dfrac{1}{4z} $ $r = \dfrac{8}{36z} + \dfrac{9}{36z}$ $r = \dfrac{8 +9}{36z}$ $r = \dfrac{17}{36z}$